1. Field of the Invention
The present invention relates to a drilling apparatus for flexible sheet made of thick thermoplastic material and more particularly to a drilling apparatus applicable for production of ordinary tightening portion made of thermoplastic woven fabric or synthetic leather, bag belt, webbing for animal, shoelace through portion and the like and not requiring any special post-treatment.
2. Description of the Related Art
According to a conventional technology disclosed in, for example, Japanese Patent Application Laid-Open No. 1-193109, in a drilling process for a printed circuit board made of epoxy resin whose substrate is glass fabric, the printed circuit board is sandwiched between a fixed fitting plate and a pressing plate fixed to a pressure foot which ascends or descends in order to reduce occurrence of burr, chipping, deviation and surface damage. Under this condition, it is intended to drill a hole having a desired diameter with using a drill. In this case, a hole is preliminarily formed in each of the fixed fitting plate and the pressing plate such that the hole has a clearance with respect to the drill.
Further, according to, for example, Japanese Patent Application Laid-Open No. 7-308896, a metallic thin plate, water-soluble lubricant soaked sheet or a sheet formed of polymer layer is placed on a multilayered substrate such as multilayered printed circuit wiring board in order to ensure heat removal effect, lubrication effect, dispersion preventing effect, and halo phenomenon preventing effect at the time of drilling a through hole by gimlet, drill or punch. Alternatively, the same substrate is sandwiched by the said sheets and then, through holes are drilled in the same substrate together with the sheets.
In recent years, many kinds of thermoplastic fiber fabrics have been used as webbing products for animal girth or strap instead of natural leather. These products have plural holes formed by drilling or punching in the same way as the drilled hole described in the above publication in order to insert a clasp of a buckle. Usually, eyelets are attached to prevent the configuration of these holes from being collapsed. However, because ordinary webbing products are woven using strict thick fibers, fibers cut on an inside face of each hole are likely to be separated loosely from its woven structure so that the eyelet drops out easily, thereby its availability being lost.
Therefore, in order to stabilize the configuration of a formed hole without attaching the eyelet thereto, a drilling apparatus which fuses an inside face of the hole by heat at the same time of drilling has been developed using thermal plasticity of composition material of the webbing products. A basic method for the drilling is to melt the inside face of the hole by heat using a heating rod or ultrasonic wave at the same time when the hole is drilled and leave the face harden.
Because the drilling apparatus described in the above publication is drilling in a multilayered substrate by rotating a gimlet or drill of 0.2 to 0.50 mm in diameter at high speeds of 50,000 to 100,000 rpm, the hole""s diameter is very small and necessarily, the position accuracy and processing accuracy are under unit of micron. Further, despite high-speed rotation and heat generation, a rotation with such speed does not generate a high temperature over a melting point of ordinary thermoplastic resin. Therefore, applying such drilling technology to the webbing products made of thermoplastic material, just as it is, is meaningless and substantially impossible.
On the other hand, if it is intended to drill a hole in the webbing products using the above-mentioned heating rod or ultrasonic wave, its processing speed is decreased and productivity also drops. Further, the inside face of the formed hole is not smooth face but there remains a melting mark and melting chips adhere to a peripheral edge of the hole, so that the peripheral edge of the hole is locally swollen and a beautiful hole""s configuration is difficult to secure.
The invention has been achieved to solve the above described problems and an object of the invention is to provide a drilling apparatus capable of obtaining a beautiful hole""s configuration with stability in a sheet, which is made of flexible thermoplastic material being different from a strict material used such as a printed circuit wiring substrate but a thick material, without special post-treatment, and ensuring a high productivity.
A sheet material which is a processing object of the invention is different from a ordinary strict plate having a low viscosity and an excellent workability such as metal and electronic parts. For example, the sheet material is composed of fiber woven fabric, fiber knitted fabric, unwoven fabric or thick thermoplastic resin material, in which thermoplastic resin is soaked, having a flexibility and difficult to machine because its viscosity is increased by a heat generated during machining.
As described above, the drilling apparatus of the invention is demanded to form a hole in the said sheet material so that the configuration of the hole is not collapsed bearing a long term use without any special treatment after drilling is made and an inside face and peripheral portion of the hole are processed beautiful.
As a result of accumulated considerations based on these premises, inventors of this invention recognized that it was necessary to develop technology, in which an inside face of the hole is melted at the same time of drilling and further no melting chips adheres to the periphery of the hole. Basically, drilling with a rotation drill which makes a sliding contact with an inside face of the hole is preferable in view of efficiency and it is preferred that the inside face of the hole can be melted at the same time.
Then, the inventors drilled in synthetic resin (nylon) being composition material of webbing products by rotating a drill of an ordinary vertical drilling machine while heating the material over a melting temperature thereof. Consequently, although an inside face of the formed hole had somewhat smoothness, melting chips adhered to the peripheral edge of the hole on a rear face in the drilling direction and further, the peripheral edge portion on the same rear face was swollen outward. Still further, because the melting resin were hardened to be adhered to blade grooves of the drill and piled, continuous drilling for a long time was disabled. The phenomenon that the melting resin were hardened to be adhered to the blade groove and piled occurred also when drilling in the front and rear faces of a drilling region of the webbing products sandwiched by a fixed fitting plate and a pressing plate as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 1-193109 by means of a heating drill. In this case, melting resin which could not escape adhered to the blade groove, so that the frequency of continuous drilling decreased by contraries.
Therefore, a conclusion that drilling with a heating drill having an ordinary configuration could not attain its practical purpose was reached. As a result of further trial processing, when a rod having a simple circular section was used by removing the blade from the drill and canceling positive heating, and feeding speed and rotation speed were set to appropriate values, the inside face of a hole was formed in equally fused smooth surface. Further, no melting chips adhered to the front and rear faces around a peripheral portion of the hole formed in the webbing tape and the peripheral portion of the hole on the rear face was not swollen. Consequently, a very beautiful hole having a stabilized configuration was obtained by accident.
Accordingly, the invention has been achieved based on the above described finding. Therefore, according to a main aspect of the invention, there is provided a drilling apparatus for flexible sheet made of thick thermoplastic resin material, comprising: a table containing a hole having a predetermined diameter at a drilling position thereof for the sheet; a drilling rod made of metal capable of rotating at high speeds while a rotation center line of the drilling rod reciprocating along a center line of the hole between inside of the hole and above the table; and a sheet pressing member disposed above the table and having an insertion hole for the drilling rod, the sheet pressing member being capable of approaching or leaving the surface of the table, wherein the drilling rod has a flat peripheral face composed of at least part of a circular section having no blade and a predetermined clearance is provided between the diameters of the hole and the insertion hole and the diameter of the drilling rod.
After the flexible sheet is placed on a table, the sheet pressing member is approached to the table and the same sheet is sandwiched with pressure between the table surface and a rear face of the sheet pressing member. Next, the drilling rod is actuated at a predetermined rotation speed and feeding speed determined depending on thermoplastic material so as to drill. At this time, frictional heat is generated between the drilling rod and sheet, so that a drilling face of the sheet in a sliding contact with the drilling rod is heated over a melting temperature and consequently melted. The melted material penetrates an inside of the sheet and a melting film is formed on the sliding contact face with the drilling rod when a hole is formed.
If the said clearance is set to an appropriate value, a sheet material melted at the time of drilling is sealed between the table and the sheet pressing member, so that the melted sheet material invades into an inside of the sheet by pressing force and centrifugal force of the drilling rod as described above. Therefore, after the drilling is completed, no melting chips swells out on the front and rear surfaces of the tape. Moreover, the inside face of the hole is adhered by the melting material and smoothed by a peripheral face of the drilling rod. Consequently, the hole is finished very beautiful and stability of the hole configuration is maintained with long-term use.
Preferably, it is specified that a peripheral speed of the drilling rod at the time of rotating is 450 to 1,000 cm/sec. The reason why not the rotation speed of the drilling rod but the peripheral speed at the time of the rotating is employed is that if the diameter of the rod is changed, a sliding speed of the drilling rod being in sliding contact with the sheet is changed. If the peripheral speed at the time of rotating of the drilling rod is smaller than 450 cm/sec, melting of the sheet material by frictional heat is not carried out, so that durability of the hole configuration drops. Further, if the said peripheral speed exceeds 1,000 cm/sec, melting width is increased so that no stabilized hole configuration can be secured.
Further preferably, it is specified that feeding speed of the drilling rod to the flexible sheet is 0.5 to 2 cm/min. This feeding speed is deeply related to pressing force to the sheet. If this feeding speed is smaller than 0.5 cm/min, melting amount on a sliding face of the sheet deviates so that processing efficiency drops and the hole configuration is not stabilized. If the feeding speed exceeds 2.0 cm/min, a strong pressing force is generated before the sliding contact face of the sheet is melted. Consequently, frictional heat largely exceeding the melting temperature is generated, so that a desired hole configuration can not obtained.
Still further, it is specified that a front end face of the drilling rod is a curved face protruded outward. And it is specified that the front end face of the drilling rod is a flat face. And it is also specified that the front end face of the drilling rod is a pair of flat faces each surrounded by part of circle and chord, the pair of the flat faces opposing each other at a predetermined distance. The inventors considered the configuration of the drilling rod in various ways to facilitate pressing of the drilling rod into the sheet. For example, the inventors considered formation of a cylindrical blade having a rod end dented inward with its periphery remaining. As a result of drilling with this rod, it was found that the hole configuration was stabilized so that it could bear drilling of several times and however, cutting chips were deposited inside the cylindrical blade at a rod end face gradually, thereby making the blade portion likely to be broken. However, this is sufficiently available for drilling of a small number of holes. This can be said for the third configuration of the front end face of the drilling rod mentioned above.
Then, as a result of accumulated considerations about the configuration of the blade, in case of using an ordinary shape of a drill, cutting chips or melting chips were piled and hardened in a pocket portion of a blade tip so that the drill could not bear a long term use. A most preferred embodiment of the invention was found by accident through the continued experiments. According to the rod configuration of this embodiment, a simple round bar having a flat front end face of the rod or a bar having a protruded semi-spherical head is employed. Consequently, in a hole formed by this drilling rod, the peripheral edge portion around the hole on the front and rear surface is very beautiful and an inside face of the hole is fused and hardened firmly. Further, a hole having a diameter of 4.6 mm can be drilled in substantially 2.0 sec in case of a webbing tape made of nylon fiber fabric, for example, although the drilling speed is different depending on material. The life of the drilling rod is quite longer than that of a conventional drill.
Preferably, it is specified that the drilling rod is comprised of a main body having the same diameter as the diameter of a hole to be formed and a protruded portion protruded from a front end of the main body along the center line and having a diameter smaller than the diameter of the main body. This rod is suitable for forming a hole having a large diameter. Upon drilling operation, first, a hole having a small diameter is formed by the protruded portion and then, a desired hole is formed subsequently by the diameter of the main body. Consequently, the peripheral edge portion around the hole both on the front and rear faces is beautiful in case of the hole having the large diameter as well as the hole having the small diameter and the inside face of the hole is fused and hardened firmly.
Further preferably, it is specified that clearance formed between a hole formed in the table and the diameters of a rod insertion hole formed in the sheet pressing member and the drilling rod is 0.1 to 0.3 mm. If the said clearance is smaller than 0.1 mm, the drilling rod is likely to interfere with the table or sheet pressing member due to vibration of the apparatus or the like. If the clearance exceeds 0.3 mm, melting chips are likely to adhere to the peripheral edge of the hole formed in the sheet, so that a stabilized beautiful hole configuration cannot be secured.